Consolidated Statements of Operations - USD ($) shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Income Statement [Abstract]
Net service revenues	$ 5,094,933	$ 4,695,014	$ 4,245,895
Direct costs of services, consisting of payroll, payroll taxes, benefit costs and reimbursable expenses	2,980,462	2,772,098	2,522,803
Gross margin	2,114,471	1,922,916	1,723,092
Selling, general and administrative expenses	1,533,799	1,425,734	1,324,815
Amortization of intangible assets	192	557	1,700
Interest income, net	(550)	(724)	(1,002)
Income before income taxes	581,030	497,349	397,579
Provision for income taxes	223,234	191,421	145,384
Net income	357,796	305,928	252,195
Net income available to common stockholders—diluted	$ 357,796	$ 305,928	$ 252,192
Net income per share:
Basic (in USD per share)	$ 2.72	$ 2.28	$ 1.85
Diluted (in USD per share)	$ 2.69	$ 2.26	$ 1.83
Shares:
Basic (in shares)	131,749	134,358	136,153
Diluted (in shares)	132,930	135,541	137,589
Cash dividends declared per share (in USD per share)	$ 0.80	$ 0.72	$ 0.64